Recently, electric vehicles, hybrid vehicles and fuel-cell vehicles have attracting attention as environmentally friendly vehicles.
Such a vehicle mounts an electric storage device such as a high-voltage battery higher than 100V, to rotate motors and driving the vehicle using power from the electric storage device.
A configuration for stepping up a power supply voltage of the electric storage device and supplying the stepped-up voltage to an inverter driving a motor in such a vehicle has been studied.
Japanese Patent Laying-Open No. 10-066383 discloses a controller for driving a permanent magnet type synchronous motor having such a voltage step-up circuit, in which necessary voltage for achieving a target operating point is computed based on torque command and motor rotation, and the voltage is stepped up accordingly.
When the operating point of the motor determined by the torque and the rotation speed is abnormal, however, it is possible that an output voltage of a step-up converter becomes lower than back electromotive voltage of the motor. In such a situation, the motor would not operate normally but might cause regenerative braking.